Two popular mistakes should be identified and avoided:

1. It is not merely ISIS against the Iraqi army. ISIS is the spearhead and the combat wise most experienced and effective single group of a variety of Sunni militias that are fighting the Iraqi armed forces. Not all of these 7 or 8 groups are radical islamists and sectarian. Many are tribal fighters disaffected with the central government which they accuse of having sidelined, oppressed and marginalized Sunnis for years. Others are former Baathists, thus more or less secular minded or nationalists, among them the Naqshbandy army.

2. Though it is true that especially the Maliki government is highly corrupt and has acted in sectarian ways, this is not merely because Malikis regime is backed by Iran or simply hates Sunnis. While ISIS as the name of a specific organization only exists since  a couple of years, the hatred ideology of takfiri salafism in post-Saddam Iraq is not that new. As early as in 2003 systematic and wide scale deadly attacks against Shia police, army recruits and especially ordinary civilians began to occur at least on a weekly basis. Suicide bombers and car bombs killed hundreds of Shia every month, targeting them in mosques, at market places, in Cafes and restaurants and even at funerals. Many Shia clerics were assassinated few months after the US invasion in 2003, e.g. Ayatollah Hakim:
http://en.wikipedia.org/wiki/Mohammad_Baqir_al-Hakim#Assassination

Not all but many instances of power abuse at the hands of Shia militias and Iraqi armed forces were a reaction to the relentless and high casualty bombings of Shia areas. Neither Iran nor the Iraqi Shia clergy brought sectarianism to Iraq. It was the „achievement“ – and not an incidental one – of Wahhabi/Salafi ideologues from the GCC countries awash in money and relying on arabic mass media in shape of several satellite channels broadcasting anti-Shia and anti-Iranian hate mongering all around the clock.

It is wrong to declare Sunni opposition to the Iraqi regime as „terrorism“ and not every Sunni insurgent fighting the Iraqi army is a takfiri. The Sunni opposition is legitimate but it suffers from being associated with ISIS and similar minded sectarian jihadists.